DHPMs produce monoastral spindles and 4p can lead cancer cells to a mitotic catastrophe phenotype. A, MCF-7 cells exposed to 4 m (1.0 mM), 4bt (dimethylenastron, 0.8 mM), 4p (0.4 mM), 4bc (1.0 mM), 4x (0.8 mM), monastrol (0.1 mM – positive control) or to culture medium only (negative control) for 24 h were immunostained with α-tubulin (green) and DNA stained with DAPI (blue). Cells were analyzed under a laser scanning confocal microscope. B, MCF-7 cells control or treated with 4p (0.4 mM) for 24 h had their nucleus stained with DAPI (left panels) or were processed and analyzed by Transmission Electron Microscopy after exposure to 4p (0.4 mM) for 48 h (right panels). Treated cells (lower panels) showed nuclear fragmentation with multiple micronuclei formation (MN), perinuclear vacuolization (V) and maintenance of cell membrane integrity (black arrows) compared to untreated cells (upper panels) showed nuclei with normal morphology and integrity (N) and (white arrowhead).